Quantum cascade (QC) lasers are powerful and compact semiconductor light sources. In the mid-infrared, they are currently highly advantageous light sources for applications in absorption-based spectroscopic systems in molecular sensing. As these systems make use of strong and narrow absorption lines of different gas molecules, they require the QC laser to operate in single-mode and be continuously tunable. Various methods of achieving wavelength selectivity and tunability have been investigated and implemented. Until recently, most single-mode operation of QC lasers has been achieved by incorporating periodic gratings on top of the regular Fabry-Perot QC lasers, such as distributed feedback gratings or distributed Bragg reflectors. However, the necessity for precise periodic structures on the wavelength scale requires complex fabrication steps, such as electron beam lithography, often resulting in higher fabrication costs and lower yield. Further, mechanically movable gratings have been incorporated into external cavities to enable wide range single-mode tuning. Nevertheless, wavelength selection and tuning in such a configuration requires system integration and stringent optical alignment. Problems, therefore, persist in providing continuously tunable single-mode QC lasers having simplified fabrication and desirable performance.